1. Field of the Invention
The present invention relates to a variable displacement wobble plate compressor for use in an automotive air conditioning system or the like and more specifically to such a compressor featuring a control arrangement which permits the output of the compressor to be selectively reduced to zero without the need of a clutch or the like to disconnect a drive connection between the compressor and a prime mover which drives the same.
2. Description of the Prior art
FIG. 1 shows a variable displacement type wobble plate type compressor disclosed in JP-A-58-158,382. This device is of the type wherein the suction pressure of the compressor is maintained essentially constant by controlling the amount of working fluid (viz., a refrigerant) discharged. This control is implemented by controlling the volume of the compression chambers of the compressor in accordance with the pressure of the refrigerant which returns to the compressor. With this arrangement when the suction pressure prevailing in the induction port of the compressor is controlled in a manner which holds the same essentially constant, the pressure of the refrigerant in the evaporator becomes essentially constant and permits exessively low evaporator surface temperatures which tend to induce freezing of water which has condensed on the evaporator, to be avoided.
The illustrated arrangement includes a drive shaft 11 which is operatively connected with an automotive engine (not shown in FIG. 1) via a pulley 2 and a magnetic clutch 2a. A drive rod 11a is provided on the drive shaft 11 and arranged to extend normally thereto. A drive plate 13 is pivotally connected to the drive rod by a pin 11b and arranged so that its angle of inclination with respect to the drive shaft 11 can be varied.
A wobble plate 16 is mounted on the drive plate 13 by way of a thrust bearing 14 and a radial bearing 15. The wobble plate 16 is provided with a shoe 19 which is slidably connected to a guide pin 18 fixed to the casing 17 of a crank chamber 12. This arrangement prevents the wobble plate 16 from rotating within the crank chamber 12 while allowing the inclination thereof to be varied.
A plurality of equally spaced piston or connecting rods 22 are connected to the wobble plate 16 by way of ball joints 22a. Pistons 23 are connected to the other ends of the connecting rods 22 by way of ball joints 22b.
When the drive plate 13 is rotated, the wobble plate is moved in a manner which induces each of the pistons 23 to undergo reciprocative movement in the cylinders or bores 26 formed in the cylinder block 25.
A cylinder head 30 is formed with an suction chamber 29 and a discharge chamber 33. The suction chamber 29 is arranged to communicate with a conduit through which refrigerant is returned from an evaporator (not shown). Inlet valves 34 operatively mounted on a valve plate 20 are arranged to control inlet ports 27 formed in a valve plate 20. Discharge valves (no numeral) also mounted on the valve plate 20 control the communication between cylinders 26 and the discharge chamber 33. The valve plate 20 is sandwiched between the cylinder head 30 and the cylinder block 25, and formed of three layers. The center layer is suitably recessed or apertured to form a communication passage structure therein as will become apparent hereinlater.
The pressure prevailing in the suction chamber 29 is supplied to an intake pressure chamber 32 by way of a connecting passage 32a. The discharge chamber 33 is communicated with a discharge pressure chamber 35 by way of a passage 35a.
A control valve Cv is disposed in a stepped bore formed in the cylinder head 30 and operatively disposed between the intake and discharge pressure chambers 32 and 35. This valve includes a first control valve element 36 disposed in the lower section of the valve body and a second valve element 39 disposed in the upper section thereof.
A bellows 37 forming part of the conrol valve Cv is arranged to expand and contract in a manner to establish an equilibrium between the pressure prevailing in the intake pressure chamber 32 and a spring 38 disposed in the bellows 37.
The first valve element 36 is operatively connected with the bellows and arranged to control the opening degree of a valve seat 40. The pressure prevailing in the intake pressure chamber 32 is introduced into the crank chamber 12 by way of the valve seat 40, passages 42 and 43, a bore 44 in which one end of the drive shaft 11 is operatively supported and an axial bore 45 formed in the drive shaft 11.
The first valve element 36 is formed with a push rod 47 which is engageable with the second valve element 39 in a manner which lifts it off its valve seat 47. The first and second valve elements 36 and 39 are arranged so that as the first valve element 36 is moved toward a closed position the second valve element 39 is moved toward an open one and vice versa.
With this arrangement when the thermal load on the evaporator is low, the refrigerant which is returned to the compressor has not absorbed much heat and thus produces a relatively low pressure in the induction chamber 29. Under these conditions the bellows 37 tends to expand and move the first and second valve elements upwardly as seen in the drawings. This movement tends to move the first valve element 36 toward a closed position and the second valve element 39 toward an open one. Accordingly, the pressure prevailing in the discharge chamber 33 tends to be supplied into the crank chamber 12 via passages 48 and 49.
The angle or amount of inclination of the wobble plate is controlled by the pressure prevailing in the crank chamber 12. When the pressure in the crank chamber increases and exceeds that prevailing in suction chamber 29, the pressure differential acting across the pistons 23 is such as to induce the situation wherein a moment of force which rotates the wobble plate 16 and the drive plate 13 about pin 11b in a direction which reduces the angle of inclination.
Under such conditions the stroke of the pistons 23 is reduced and the amount of refrigerant discharged by the compressor is correspondingly reduced. As the amount of refrigerant circulated through the system is reduced the amount of heat absorber per unit volume of the refrigerant increases and the pressure prevailing in the suction chamber 29 is gradually increased and an essentially constant induction pressure is resultingly maintained.
When the thermal load on the evaporator is high, the pressure in the suction chamber 29 increases. This induces the bellows 37 to contract and cause the second valve element 39 to move toward a closed position and the first one 36 to move toward an open one.
Under these conditions the pressure prevailing in the crank chamber 12 tends to become equal to that in the suction chamber 29. Further, as the pressure differential across the pistons 23 during their respective suction strokes is essentially zero, the pistons are permitted to move unrestrictedly toward the end of their bores. This, in combination with the counterforce generated during the compression strokes produces a moment of force which tends to rotate the wobble plate 16 in a manner which increases its degree of inclination. This increases the amount of refrigerant discharged by the compressor and increases the amount of refrigerant which is circulated through the circuit including the evaporator. This lowers the pressure prevailing in the suction chamber and again results in an essentially constant suction pressure being maintained.
The above described arrangement has encountered the drawback that if the wobble plate is permitted to assume a position wherein it is normal to the drive shaft 11, no backpressure is developed and it is difficult to get the compressor to again discharge refrigerant.
Accordingly, a spring 50 has been provided which ensures that upon start-up the wobble and drive plates 16, 13 will be biased sufficiently to ensure that refrigerant discharge can always be induced when required. However, provision of this spring 50 requires that the clutch 2a be provided to enable the compressor output to be selectively reduced to zero.
The provision of this clutch 2a which includes an iron core, coils, roller bearings and the like, complicates the constuction and increases the weight of the device as a whole. This increases both the production costs and the fuel consumption of the vehicle to which it is applied.